The invention relates to a charging method for batteries, in particular for NiCd (nickel-cadmium) and NiMH (nickel hydride) cells, in which the batteries are supplied from a charging current source.
A number of such charging methods are known to date (cf. "Intelligent charging of NiCd batteries" from Design und Elektronik 22, 1992, pages 106-107), all of which are based on the objective of providing a low-cost, optimum battery charging concept. The intention is to ensure that the battery is charged as nearly as possible to its rated capacity, while at the same time avoiding overcharging, which could shorten its service life. The charging time also constitutes a criterion for evaluating the process, in that prolonged charging periods are no longer accepted. On the one hand, rapid charging with high-strength current considerably reduces the charging time. On the other hand, however, the charging period must then be precisely adhered to, otherwise the battery may be damaged beyond use. This means that a battery charger is required with a control unit which constantly monitors the charming current and the state of charge of the battery. Thus, various processes are known which monitor the dU/dt pattern of the charging curve, from which a switch-off criterion is derived. Typically, the charging process can be terminated when the battery voltage falls, i.e. with a negative du/dt value, or even with a pronounced increase in the battery voltage, i.e. with a positive dU/dt value. Up to now, the monitoring of the voltage gradient in a charging curve of this type has only been carried out with the aid of considerable circuitry resources, which in turn are reflected in high-cost batteries.